(1) Field of the Invention
The present invention relates to a heat-sensitive recording element. More particularly, the present invention relates to a heat-sensitive recording element in which a starch partially esterified with an organic carboxylic acid is used as a polymeric binder for dispersing and binding a leuco pigment and an organic acidic substance.
(2) Description of the Prior Art
The silver salt photographic process, the diazo photographic process, the electrophotographic process and other electric recording processes have heretofore been utilized as means for recording informations. Recently, the thermographic recording process has been noted as the recording process in which a visible image can be directly obtained without performing development and fixation treatments.
A recording element comprising a recording layer formed on a substrate, said recording layer comprising a leuco pigment which is colorless or light-colored in the normal state, an organic acidic substance which is solid at normal temperatures but is fusible under heating, and a polymeric binder in which said leuco pigment and organic acidic substance are dispersed, is broadly used as the recording element for the thermographic recording process. Water-soluble substances capable of dispersing the organic acidic substance and leuco pigment therein without dissolving them are ordinarily used as the polymeric binder. Various starches and starch derivatives such as hydroxyethyl starch, hydroxypropyl starch, carboxymethyl starch, oxidized starch and soluble starch have been known as polymeric binders which meet the above requirement and are available at low costs.
However, certain defects are commonly observed when these starches and starch derivatives are used. For example, since starch or starch derivative is a nutrient for microorganisms, an aqueous solution of a starch or starch derivative readily gets moldy when the aqueous solution is stored. Most of starches and starch derivatives are easily soluble in hot water, but hot water solutions of starches or starch derivatives are readily gelatinized when they are cooled. When a starch derivative not having such tendency is used, the water resistance of the resulting recording layer is extremely poor, and if the recording layer falls in contact with water, flow-out or bleeding of the image is readily caused. Moreover, starches and starch derivatives which have been heretofore used in this field are not satisfactory in such properties as the property of dispersing and binding a leuco pigment and an acidic substance, the easily defoaming property and the water resistance.